A patch of bumps on the arm upon scratching may be caused by conditions such as pruritus, artificial urticaria, scabies and eczema. 1. Pruritus: Pruritus patients will have itchy skin, may be related to clothing irritation or dry skin, the skin will be itchy, the patient scratching, the area may appear red spots or pimples. 2. Artificial urticaria: artificial urticaria belongs to one kind of urticaria, the typical symptom is that the skin will appear itchy after being stimulated, after the patient scratches, there will be erythema in the area or a rash like a wind ball, which can generally subside on its own. 3. Scabies: This is caused by scabies infection, usually occurring on the vulva, finger cracks, limbs flexion and other tender skin parts, itching at night, patients repeatedly scratching, the affected areas may appear pimples or even scabies nodules. 4. Eczema: eczema has more causes, there will be obvious itching symptoms, scratching the affected area, the site will appear in varying sizes of pimples or erythema, acute eczema may appear vesicles, oozing, and the formation of yellowish scabs, repeated scratching for a long time can also form chronic eczema, manifested as thickening of the skin, roughness and so on. Many diseases can lead to arm scratching a pimple is also seen in other causes, it is recommended that patients seek medical attention in a timely manner, to do a systematic examination, to clarify the cause of the disease.